LOW-TEMPERATURE (4.2 K) NA-23 AND AL-27 SWEPT-FIELD NUCLEAR-MAGNETIC-RESONANCE SPECTROSCOPY OF ZEOLITES - OBSERVATION OF FRAMEWORK AND NONFRAMEWORK ALUMINUM SITES

A new method for acquiring Na-23 and Al-27 nuclear magnetic resonance (NMR) spectra of zeolites is introduced. The key features are operatio n at low temperature, 4.2 K, and the ability to scan across large spec tral regions. In this method, the magnetic held is slowly swept throug h the resonance condition as the line shape is recorded by a low-power single pulse or spin echo pulse sequence. A single crystal and powder sample of andalusite are studied to verify the spectral response feat ures of the pulse sequences. Then, four hydrogen faujasites are studie d; the samples have different amounts of non-framework aluminum. By ac quiring the spectra at 4.2 K, the motion of ions and water molecules i s suppressed and the NMR spectra are obtained in the static limit. We compare these spectra to simulations based on Al-27 quadrupole couplin g constants previously obtained in room temperature studies. There is partial agreement, but the 4.2 K spectra appear to show more Al-27 sit es than are observed at room temperature.